I have a hard time believing this – they only went into beta a week or two ago.
Still, if its true, this would be the first commercial implementation of Unlicensed Mobile Access. This service allows subscribers to use either cellular service or WiFi service to place & receive phone calls.
With T-Mobile having such an extensive WiFi Hotspot network, in addition to supporting free access points, and in conjunction with their GSM network, would allow users to make inexpensive calls many additional locations.